running more power through the poor mobo's traces is asking a bit much. The external power plugs will be the way to go.
150watts/12V = 12.5A... thats a lot to run through the mobo
( on a 6 pin cable the minifit molex connectors are rated for around 9A each but the wire if its a 16ga can handle between 13A(enclosed) and 22A(in free air) each. So if you go with the lower spec (minifit ends) a 6 pin cable for a GPU can supply about 27A (27A X 12V=324 watts. I havent played around with the 8pin plugs too much but I noticed they are the same as the 6 pin ones (3 yellows) they just add 2 extra grounds to them.
Thats exactly where i was going to go as well- im going back into electronics history a bit here, but in the early days of the connector that evolved into the pci and pcie connectors of today, in the broadcast and audio recording fields were "edge card connectors".
Broadcast manufacturers like gates (harris allied) QRK and leagues of others all jumped on that technology in the late 60's early 70's and proceeded to guarantee employment for a lot of folks like me. They would build an entire console and transmitter parts as well based on this and if they needed more current just feed extra pins in paralell. new problem, too damn much heat, pins lose thier tensile strength, card pins start arcing, copper starts burning boards. and what once was the smell of success turns into a foul stench that means money in my pocket....
Next time you get to see the bottom of a mobo take a good look a how thin some of the traces are feeding that pci slot, and then there are feedthru points along the way that create more problems in a high current application.
When i got my first 780 chipset board i noticed in bios that you could assign a max wattage of 150w on the biostar bios for the pci slot and i just shuddered to think what would actually happen if a card tried to go there all on the slot power.
they have no choice, thats all an edge card connector will ever hold. but i bet we will see 8 pin molex with 4 b+ and 4 ground as the norm by 2012. just watch, its the only way unless they start making some real efficiant gpus with cooling that hasnt been thought of yet.
so its not so much because of backwards compatibility, but thats all those pins and copper can hold mathmatically and nothing can change that except for a radical new design in circuitbords and connectors, but not with things in existence as we know them today.
keep in mind 75 watts is considered average useage, and 150 is peak MAX , if your card tried to go to max for anything more than a few seconds hold time, it would not take long before you would see smoke-you can bank on that.